In the past, various techniques have been developed for the purpose of sterilizing or decontaminating biological sludges and wastes. The most common process is the process of mixing lime with the sludge. The reaction of lime with the water in the sludge serves to elevate the temperature of the sludge to a maximum of 100.degree. C.
In the past, various U.S. patents have issued relating to processes for the decontamination and treatment of wastewater sludges. For example, U.S. Pat. No. 4,038,180, issued on Jul. 26, 1977 to N. K. Talbert, describes a process of dewatering sewage sludge in which the sludge from a municipal or industrial sewage treatment facility is mixed with a mineral acid or anhydride thereof to release the entrapped water in the sludge. The resulting mixture of the sludge solids and diluted acid or anhydride is then mixed with a basic material, such as ammonia, such that the heat generated by the reaction of the base and the acid evaporates the water to form either a completely dry mixture of sludge solids and a salt or a mixture having a predetermined moisture content which may be air dried.
U.S. Pat. No. 4,500,428, issued on Feb. 19, 1985 to Lynch et al., describes a method for the treatment of a wastewater sludge using a pair of reaction vessels, sequentially, to treat the sludge. Both of the vessels are pressurized. The first vessel has an aerator for aerating the sludge. This vessel receives sulfuric acid and chlorine therein through a port. A dewatering device is provided upstream of the first vessel. The outlet of the first vessel is coupled to an inlet of the second vessel through another dewatering device. The second vessel creates a final-treatment chamber in which the sludge is exposed to ozone, air and lime.
U.S. Pat. Nos. 4,781,842 and 4,902,431, issued to Nicholson, teach processes for the decontaminating of wastewater sludges to a level which meets or exceeds U.S. E.P.A. process standards. The process mixes sludge with an alkaline material sufficient to raise the pH of the end product to 12 or higher for at least one day. This process will raise the temperature to 50.degree. C., but will not sterilize the sludge nor does it eliminate the pathogenic organisms.
U.S. Pat. No. 4,306,978, issued to Wurtz, relates to a process of lime stabilization of wastewater treatment plant sludge. This patent discloses the dewatering of the sludge and intimately mixing calcium oxide to raise the temperature so as to produce a stabilized sludge particle.
U.S. Pat. No. 5,482,528, issued on Jan. 9, 1996 to Angell et al., teaches a pathogenic waste treatment process for the processing of solid waste and for the converting of such solid waste into useful products. This is accomplished by combining the waste with an acid, such as concentrated sulfuric acid, and a base, such as fly ash. These exothermically react and thermally pasteurize the waste and add mineral value to the product. Pozzolanic materials, such as fly ash, agglomerate the product. The calcium oxide in the fly ash reacts with sulfuric acid to form calcium sulfate dihydrate.
None of these prior art patented processes are capable of achieving temperatures, when mixed with the sludge, of greater than 100.degree. C. None of the prior art techniques allow for the shorter drying times as required by 40 C.F.R. Subchapter O, Part 503.32.
U.S. Pat. No. 5,635,069 issued on Jun. 3, 1997 to the present inventors. This patent described a process for treating a waste sludge of biological solids which included the steps of mixing the sludge with an oxide-containing chemical and sulfamic acid so as to elevate the temperature of the sludge, pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.a., and discharging the pressurized mixed sludge. The oxide-containing chemical could be either calcium oxide, potassium oxide, or potassium hydroxide. The sludge has a water content of between 5 and 85 percent. The oxide-containing chemical and the acid are reacted with the sludge so as to elevate the temperature of the sludge to between 50.degree. C. and 450.degree. C. The pressurized mixed sludge is flashed across a restricting orifice or passed into a chamber having a lower pressure. The evaporated liquid component can be condensed and used as part of the process or external of the process.
Experiments with the process of this prior art patent have disclosed that this process is extremely effective in the treatment of waste sludges. However, certain improvements were found possible with the process of U.S. Pat. No. 5,635,069 which renders the process more economic and more assuredly pathogen-free. In particular circumstances, it was found that the cost of the oxide-containing chemical could be replaced, in certain environments, by applying heat directly to the pressurized sludge. In other circumstances, if heat is applied directly to the pressurized mixed sludge, then the sulfamic acid could be replaced by less expensive chemicals, such as carbon dioxide. In certain circumstances, it was found that the use of electricity for the heating of the pressurized sludge was less than the cost of certain chemicals used to elevate the temperature of the sludge.
It is important to note that in existing processes for the treating of waste sludges, it is common to use a conveyor belt onto which the dewatered sludges are placed. This conveyor belt will pass the dewatered sludges under infrared radiation so as to effectively heat the sludge. Unfortunately, the use of such radiant energy for the heating and pathogen destruction of the waste sludge is extremely unefficient. In other cases, the application of such infrared energy to the waste sludge will cause serious odor and toxicity problems. It was found that the use of such infrared radiation for the treating of pathogens in the waste sludge had an energy efficiency of only 18 percent with respect to the amount of heat that could be applied to the waste sludges. As such, a need developed for the cost-effective and efficient pathogen-destruction and heating of the waste sludge so as to conform with E.P.A. requirements.
It is an object of the present invention to provide a process for rendering a biological waste sludge pathogen-free and vector-free.
It is another object of the present invention to provide a process that converts the biological waste sludge into a sellable end product.
It is a further object of the present invention to provide a process that eliminates or reduces waste incineration and landfilling of waste sludges.
It is another object of the present invention to provide a process which reduces the odor emitted during the heating of the waste sludge.
It is still a further object of the present invention to provide a process that efficiently utilizes energy for the heating of the waste sludge.
It is a further object of the present invention to provide a process that is adaptable for optimizing the cost of chemicals used for the treatment of the waste sludge.
It is still a further object of the present invention to provide a process for the treating of waste sludges that is cost effective, easy to use, and easy to install.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.